This invention relates to a device for driving a laser with a digital input signal, comprising:
quiescent-current means for applying to the laser a quiescent current which is controlled by a negative feedback current which is proportional to the light output of the laser in order to stabilize the light output of the laser at a level which corresponds to one of the two possible values of the digital input signal,
a modulation device for applying to the laser a drive current modulated by the digital input signal, and
a compensation device for applying a compensation current to the quiescent current means in order to compensate for a d.c. component of the drive current, which component varies with the duty cycle of the digital input signal.
Such a device is suitable for use in optical transmission systems and is very suitable for use in digital optical telecommunication systems.
Such a device is known from "The Bell System Technical Journal", Vol. 62, Sept. 1983, No. 7, Part 1, pages 1923-1935. The driving characteristic of an injection laser usually has a first range of gradual slope in which the light output of the laser increases only slightly as the drive current increases and which above a specific threshold value changes into a second range of steeper slope in which the light output increases substantially as the drive current increases. Generally the quiescent-current means ensure that the laser is operated near the threshold value so that, at the operating point, the light output of the laser corresponds to a logic "0" of the digital input signal. In the case of a logic "1" of the input signal, the drive current supplied to the laser by the modulation device then switches the light output of the laser to a higher level. In order to stabilize the light output corresponding to logic "0" with respect to variations in and changes of the laser characteristics, the quiescent current of the laser is controlled by a negative feedback current which is proportional to the light output of the laser and which is obtained by means of a photodetector. However, this negative feedback also eliminates the low-frequency component of the drive current. In order to prevent a digital input signal comprising mainly logic "ones" from driving the laser below its threshold value, a compensation current is added to the quiescent current in the known device, which compensation current is proportional to the drive current from the modulation device. In the case of an input signal comprising many "ones", this compensation current causes the operating point of the laser to increase from a point near the threshold value to a point where the light output of a laser substantially corresponds to that of a logic "1".
However, a drawback of the known device is that the current which is required for compensation and which is proportional to the quiescent current cannot be tapped from this drive current because this would result in the drive current for the laser being adversely affected by the compensation device. For generating the compensation current the known device comprises a second modulating device which, like the first modulation device, is controlled by the digital input signal. This second modulation device, however, requires the use of a second high-frequency circuit in the device, which demands a critical design of the circuit in order to prevent both the drive current and the compensation current from being disturbed. Therefore, it is an object of the invention to provide a device for driving a laser which does not require the high-frequency part of the circuit to be extended in order to generate the compensation current. In accordance with the invention, a device of the type defined in the opening paragraph is characterized in that
the quiescent-current means stabilize the light output of the laser at a level corresponding to the high value of the digital input signal,
the modulation device comprises a converter for converting the digital input signal into two drive currents having signal components of equal amplitudes but in phase opposition, and
one drive current applied to the laser input and the other drive current is supplied to a control input of the compensation device. In the device in accordance with the invention the quiescent current ensures that the laser is not operated at a point where the light output corresponds to a logic "0", but at a point where the light output corresponds to a logic "1". In the case of a logic "0" a compensation current is added to the quiescent current to lower the operating point to a point near the threshold value of the laser. The compensation current required for this purpose is obtained simply by constructing the modulation device as a convertor having one output that supplies the drive current for the laser and with its other output supplying a current which is equal but in phase opposition to the drive current. A fraction of the latter current is now employed as the compensation current. The drive current and the compensation current are generated by a single modulation device so that it is not necessary to extend the high-frequency part of the device in order to obtain the compensation current.
A simple and suitable converter can be obtained in an embodiment which is characterized in that the converter is a differential amplifier comprising two emitter-coupled transistors whose bases are coupled to input terminals for receiving the digital input signal and whose collectors are coupled to output terminals for supplying said one and said other drive current.
A further embodiment may be characterized in that the compensation device is an operational amplifier having an inverting input, a non-inverting input and an output. The inverting input is coupled to a first resistor for converting the other drive current into a voltage. The output is coupled to the base of a transistor whose collector is coupled to a second resistor and to the non-inverting input of the operational amplifier. The emitter of this transistor is coupled to an output for supplying the compensation current. If the first resistor is a variable resistor, this makes it possible to adjust the magnitude of the compensation current and hence of the light level corresponding to a logic "0".